The present invention relates to a method for driving a gas discharge display panel. More particularly, it relates to an improved method for driving an AC driven plasma display panel (hereinafter PDP) wherein a fired cell is erased, or extingished, by a cancelling signal having a low peak voltage value.
The most typical type of gas discharge display panel is a PDP, wherein arrays of parallel conductors, typically designated X-electrodes and Y-electrodes, are disposed on opposite sides of a gas filled panel and oriented at transverse angles to each other, forming a matrix type arrangement. The electrodes are insulated from direct contact with the gas by a layer of dielectric material. Individual discharge sites (cells) located at coordinate intersections on the panel defined by corresponding X- and Y-electrode pairs, are selectively fired by application of write signals to the respective electrodes, providing a potential across the cell which exceeds the firing potential for producing a gas discharge in each such cell. Alternating sustain pulse signals, of lower amplitude than the firing potential, are continuously applied to all the cells, and sustain discharges in the fired cells due to a wall potential, or wall charge, which develops on the panel surface of each cell undergoing discharge, and which is in additive voltage relationship to the sustain pulse signals applied to the cell. The light emitted from the selectively fired cells forms a desired display.
A fired cell is erased by applying thereto an erasing signal, comprising a pulse having the same peak voltage as that of the sustain signal but of much reduced time duration (i.e., a "narrow" pulse width), which neutralizes the wall charge; this prevents the subsequent discharge of the cell, since in the absence of the wall charge, or potential, the sustain voltage is insufficient, by itself, to maintain the discharge.
There are known various PDP drive systems and corresponding driving methods. In one known system, addressing a cell is performed by simultaneously inputting half-amplitude write (or erase) signal voltages to the associated X- and Y-electrodes. Such half-selection systems, however, require complicated drive systems for both the X- and the Y-electrodes and tend to produce misfirings. There are also known PDP drive systems and corresponding methods which do not adopt the half-selection techniques; however, such systems employ complicated driving circuits for performing the addressing function and various power sources, resulting in a significant cost increase for the driving circuit. In addition, the required operating voltages for the driving signals are rather high. For example, in one such system, the peak voltage values of the required write, sustain, and erase signals are 140 volts, 90 volts, and 90 volts, respectively. As a result, there has been a substantial effort made to both reduce the number of the required power sources and to lower the required peak voltage values of the driving signals, so as to reduce the cost of the driving circuits. Particularly, the reduction of the peak voltage of the signals allows the use of elements, such as transistors, having a relatively low breakdown voltage, resulting in a substantial cost reduction of the semiconductor integrated circuits which are used.
The advantages of the present invention may be made more apparent, by comparison to an example of a prior art PDP driving method employing signals having waveforms as are illustrated in the time charts of FIG. 1. The waveform of each of the signals is rectangular. Diagonal lines drawn in the rectangles represent that the signals are applied selectively. The voltage actually applied to a given cell C.sub.ij of a PDP is determined by the difference of the signals applied selectively to the associated electrodes, i.e., X.sub.i -electrode and the Y.sub.j -electrode, by external drive circuits. With reference to FIGS. 1(a) and 1(c), there are applied to the Yj-electrode of the PDP, a write signal 52 of approximately 140 V, an erase signal 53 of approximately 90 V having a narrow pulse width, and repetitive sustain signals 51 of approximately 90 V, all of the same polarity. In the foregoing and hereafter, reference to the "value" or "level" of the voltage of a (pulse) signal means, in every case, the peak voltage of the (pulse) signal. Further, in this prior art PDP driving method, there are applied to the X.sub.i -electrode, as seen in FIG. 1(b), a write signal cancelling pulse 55 of approximately 90 V, a preparatory converting signal 56 of approximately 90 V, and a train of repetitive sustain signals 54 likewise of approximately 90 V. As more fully described in U.S. Pat. No. 3,771,016, Toba et al., issued Nov. 6, 1973 and assigned to the common assignee of the present invention, for erasing any given cell, the cell is addressed by a related pair of signals comprising a preparatory signal and a subsequent erase signal; writing or nonwriting (i.e., maintaining a cell in a current state) for any given cell is performed by applying simultaneously to the respective X- and Y-electrodes, a combination of a write signal and the absence or presence, respectively, of a write signal cancelling pulse.
More particularly, the operation of a cell by the pulse signals as above described is performed as follows, with reference to the time charts of FIG. 1(a) through 1(f). When a cell C.sub.ij is selected to be fired, (i.e., the write function), a write pulse 52 of 140 V is applied to the Y.sub.j -electrode, as shown in FIG. 1(a) and no write signal cancelling pulse is applied to the X.sub.i -electrode, i.e., the write signal cancelling pulse 55 shown by dotted lines in FIG. 1(b) is not applied. There results a cell potential 52 of 140 V as shown in FIG. 1(c), which is sufficient to fire the cell. The cell discharge thereafter is sustained by the subsequent, alternating sustain signals 51 and 54 until an erasing process is applied to the cell C.sub.ij for extinguishing the discharge. When the cell C.sub.ij is not to be fired, a write signal cancelling pulse 55 of 90 V (shown in dotted lines in FIG. 1(b)) is applied to the X.sub.i -electrode, reducing the cell potential to 50 V (i.e., 140 V-90 V), shown by the dotted line 55' in FIG. 1(c), the cell potential 55' of 50 V thus being much lower than the firing voltage of any given cell C.sub.ij.
For erasing the cell C.sub.ij, once fired, a preparatory converting signal 56 of 90 V is applied in advance to the X.sub.i -electrode, as shown in FIG. 1(e), which converts the polarity of the cell wall potential; thereafter, an erase signal 53 is applied to the Y.sub.j -electrode as shown in FIG. 1(d), producing the cell potential shown in FIG. 1(f). As a result, the discharge in the cell extinguishes, thus erasing the prior discharge display at the cell.
In the prior driving method described above, drive systems for driving Y-electrodes and X-electrodes are required to output, selectively, pulse signals having a sequence of voltage levels of 90 V, 140 V and 90 V, respectively. The relatively high level of these voltages contributes to the corresponding, relatively high costs of the drive circuits since they must employ several transistors and other elements, all having high breakdown voltage characteristics. Further, because of various electrical and physical conditions and characteristics of the individual cells of a PDP, there is an inherent problem arising out of the differences in the gas discharge characteristics of the individual PDP cells, throughout the two dimensional array of cells of the PDP. For example, there are differences, from cell to cell, in the time delay between the application of a potential sufficient to fire a cell and the actual initiation of the cell discharge. On the other hand, there is typically only a very short delay interval, such as one microsecond, between the application of the appropriate signals to erase the gas discharge in a given cell and the termination of the discharge. A driving method utilizing these characteristics of the cells of a PDP is disclosed in Japanese Pat. No. SHO-49-38848 of Umeda and Toba, published Oct. 21, 1974; in accordance with that method, all of the cells located on a given row, or in a given column (and thus corresponding to all the cells on an X-electrode or Y-electrode, as disclosed herein) initially are fired simultaneously and, immediately thereafter, the cells which are not to be fired are erased selectively by appropriate erase signals. This method can serve to overcome the non-uniformity of the gas discharge characteristics of the cells of a PDP and thus increase the operating voltage margin for the respective signals.